Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227 (“the '227 patent”), issued Feb. 4, 2003 in the name of Paul Meadows et al., which is incorporated herein by reference in its entirety.
Spinal cord stimulation is a well-accepted clinical method for reducing pain in certain populations of patients. As shown in FIGS. 1A and 1B, a SCS system typically includes an Implantable Pulse Generator (IPG) 100, which includes a biocompatible case 30 formed of  titanium for example. The case 30 holds the circuitry and power source or battery necessary for the IPG to function. The IPG 100 is coupled to electrodes 106 via one or more electrode leads (two such leads 102 and 104 are shown), such that the electrodes 106 form an electrode array 1 10. The electrodes 106 are carried on a flexible body 108, which also houses the individual signal wires 112, 114, coupled to each electrode. The signal wires 112 and 114 are connected to the IPG 100 by way of an interface 115, which may be any suitable device that allows the leads 102 and 104 (or a lead extension, not shown) to be removably connected to the IPG 100. Interface 115 may comprise, for example, an electromechanical connector arrangement including lead connectors 38a and 38b configured to mate with corresponding connectors 119a and 119b on the leads 102 and 104. In the illustrated embodiment, there are eight electrodes on lead 102, labeled E1-E8, and eight electrodes on lead 104, labeled E9-E16, although the number of leads and electrodes is application specific and therefore can vary.
The electrode array 110 is typically implanted along the dura of the spinal cord, and the IPG 100 generates electrical pulses that are delivered through the electrodes 106 to the nerve fibers within the spinal column. The IPG 100 itself is then typically implanted somewhat distantly in the buttocks of the patient.
Further details concerning the structure and function of typical IPGs and IPG systems are disclosed in U.S. patent application Ser. No. 11/305,898, filed Dec. 14, 2005, which is filed herewith via an information disclosure statement and which is incorporated herein by reference.
IPGs are active devices requiring energy for operation, such as is typically provided by a battery. It is often desirable or necessary to recharge the battery within an IPG via an external charger, so that a surgical procedure to replace a power-depleted implantable pulse generator can be avoided. To wirelessly convey energy between the external charger 12 and the IPG 100, and as shown in FIG. 2A, the charger 12 typically includes an energized alternating current (AC) coil 17 that supplies energy 29 to a similar charging coil 18 located in or on the IPG 100 via inductive coupling. In this regard, the coil 17 within the external charger 12 is wrapped so as to lie substantially parallel to the plane of the coil 18 within the implantable medical device during charging. As shown, and as is well known, such a means of energy 29 transfer can occur transcutaneously, i.e., through the patients tissue 25. The energy 29 received by the IPG's coil 18 can then be stored in a rechargeable battery 26 within the IPG 100, which can then be used to  power the electronic circuitry that runs the IPG 100. Alternatively, the energy 29 received can be used to directly power the IPG's electronic circuitry, which may lack a battery altogether.
As shown in FIGS. 2A and 2B, an IPG 100 typically includes an electronic substrate assembly 14 including a printed circuit board (PCB) 16, along with various electronic components 20, such as microprocessors, integrated circuits, and capacitors, mounted to the PCB 16, as well as the coil 18. Ultimately, the electronic circuitry performs a therapeutic function, such as neurostimulation. The IPG 100 further includes a plastic insert 23 with a retainer 22 for holding the electronic substrate assembly 14 in place. A feedthrough assembly 24 routes the various electrode signals from the electronic substrate assembly 14 to the lead connectors 38a, 38b, which are in turn coupled to the leads 102 and 104 (see FIGS. 1A and 1B). The IPG 100 further comprises a header connector 36, which among other things houses the lead connectors 38a, 38b. The IPG 100 can further include a telemetry antenna or coil (not shown) for receipt and transmission of data to an external device such as a hand-held or clinician programmer (not shown), which can be mounted within the header connector 36. As noted earlier, the IPG 100 usually also includes a power source, and in particular a rechargeable battery 26, which may be mounted in place by other retainers formed as a portion of the plastic insert 23.
As also noted earlier, the IPG 100 also includes a case 30, which serves to house all of the aforementioned components in a suitable manner. In particular, the case 30 comprises two case halves 32, 34 that mate with each other in a clam-shell arrangement to hermetically seal the IPG 100. The case 30 has a top surface 40, a bottom surface 42, and an edge 44 between the top and bottom surfaces 40, 42.
As can be seen from FIG. 2A, the charging coil 18 according to conventional wisdom is situated adjacent the top surface 40 of the case 30 to minimize the attenuation of the energy 29 before it is received by the charging coil 18. That is, assuming that the IPG 100 is implanted within the patient such that the bottom surface 42 faces away from the external charger 12 and the top surface 40 faces towards the external charger 12, the distance D that the energy 29 must travel before it impinges on the charging coil 18 will be minimized, which in turn maximizes the efficiency of the power transmission between the two coils 17 and 18. In addition, such top-sided placement of coil 18 requires the energy 29 only to traverse the case 30, and no other components, before it reaches the charging coil 18, which again minimizes energy 29 attenuation. While according to this conventional wisdom it is preferred to place the coil 18 as  near the top surface 40 as possible, in reality it can be expected that the center of the coil is within the top 25% of the thickness T of the case (i.e., ΔT <25% of T).
However, while such top-sided placement of the coil 18 within the IPG 100 has been preferred to minimize the distance D, and hence to minimize energy 29 attenuation, such a design is met with other problems.
First, because the charging coil 18 is located closely adjacent the wall of the case 30, the case 30 may electrically interact with the charging coil 18, thereby degrading the performance of the coil 18.
Second, the edge 44 of the case 30 has a curved surface 46, resulting from manufacturing limitations as well as the clinical desire to avoid sharp edges that may otherwise irritate or damage the tissue 25 surrounding the IPG 100. (The header connector 36 likewise has curved surfaces to avoid sharp edges). To locate the charging coil 18 adjacent the top surface 40 of the case 30, the charging coil 18 must necessarily be placed at the curved surface 46 of the edge 44. Unfortunately, this limits the lateral size of the coil 18, and as best shown in FIG. 2B, works a loss of lateral distance ΔL within the IPG case 30. This loss of lateral distance means that each turn of the coil encompasses a smaller area, which in turn limits the gain of the coil 18. Therefore, while placing the coil toward the top surface 40 of the case increases the gain from the perspective of minimizing the distance D from the external charger 12, it reduces the gain from the perspective of coil 18 area.
Third, when implanting the IPG 100, the physician must ensure that the top surface 40 of the case 30 faces towards the external charger 12. If the physician accidentally flips the IPG 100 during implantation such that it is in an improper top-down configuration, the external charger 12 will not be able to as effectively communicate with the IPG, since the coil 18 will be facing in the opposite direction away from external charger 12. (It has been suggested that the incidence of flipped IPG during implantation due to physician inadvertence may be on the order of 3 to 5%). Moreover, in some cases, the IPG 100 may be properly oriented when initially implanted, but then inadvertently flipped within the patient's tissue 25, such as by the patient “fiddling” with the IPG through his or her skin. Regardless of the reason, if the IPG 100 is inadvertently disoriented in the patient with in an improper top-down configuration, the power transfer efficiency benefits realized from placement of the coil 18 toward the top surface 40 of the IPG 100 are lost. 
FIG. 3 illustrates another example of an IPG 50 capable of wirelessly receiving energy from an external charger 12 via inductive coupling. The IPG 50 is similar to the previously described IPG 100, with the exception that the charging coil 52 resides on the top surface of the case 54. To so mount the charging coil 52, the case 54 is encapsulated with a suitable biocompatible material 56 (e.g., epoxy), which holds the charging coil 52 in place. Thus, it can be appreciated that the charging coil 52 can be located even closer to the external charger 12, and the attenuation effect of the case 54 can be eliminated, thereby making energy 29 transfer between the external charger 12 and pulse generator 50 more efficient. Moreover, in this embodiment, because the coil 52 is on the outside of the case 54, its location is more readily apparent, making it less likely that an implanting physician would inadvertently implant in an improper top-down configuration.
There are, however, drawbacks to the design of FIG. 3. In particular, placement of the charging coil 52 on the exterior surface of the case 54 and the addition of the encapsulating material 56 increases the overall thickness of the pulse generator 50, thereby making the implanted pulse generator 50 more noticeable to the patient. Also, additional feedthrough holes must be made through the case 54 to connect the charging coil 52 to the electronic circuitry contained within the case 54, thereby increasing the design complexity and cost of the pulse generator 50.